The present invention relates to a method for fabricating/repairing an integrally bladed rotor and a novel encapsulated local airfoil heating device used in the method.
The increasing use of integrally bladed rotor hardware in large, high performance gas turbine engines is driven by the demand for improvements in performance and efficiency. In conventional rotors, rotating airfoils are retained by dovetail slots broached into the rim of a disk. In an integrally bladed rotor, the airfoils and disk form one continuous piece of metal. The weight and fuel savings afforded by integrally bladed rotors result from their ability to retain rotating airfoils with less disk mass than would be required in a conventionally designed rotor. Furthermore, the reduced disk mass of an integrally bladed rotor disk permits weight reduction in other components which react upon or obtain a reaction from the rotors, i.e. shafts, hubs, and bearings.
In the past, a major disadvantage associated with the use of integrally bladed rotors in large gas turbine engines has been the lack of a reliable method for repairing integrally bladed rotor airfoils that have been damaged beyond blendable limits. Because the airfoils are integral with the disk, damage to airfoils beyond blendable limits requires the removal of the entire rotor from service and replacement with a new integrally bladed rotor, at significant expense.
Other problems associated with integrally bladed rotors relate to the fabrication method employed to manufacture them. They can be machined out of a single large forging; however, this approach is not desirable. A large forging, e.g., large billet, has lower property capability and a significant amount of the starting material, which can be very expensive depending upon the material, is machined away. Also, the part is at risk of scrap out due to inevitable machining errors during manufacture. Another approach for manufacturing integrally bladed rotors is to attach separately forged airfoils to a rotor by a bonding process.
A titanium alloy consisting essentially of 6.0 wt. % aluminum, 2.0 wt. % tin, 4.0 wt. % zirconium, 6.0 wt. % molybdenum, and the balance essentially titanium is a desirable alloy for integrally bladed rotors due to its high toughness, its tensile and fatigue strength, and its good weldability. It is however a difficult alloy to process after welding because of the nature of the weld zone microstructure which is an orthorhombic martensite. First, the OEM friction weld must be post weld heat treated to stabilize the microstructure and relieve stresses. Secondly, the integrally bladed rotor must be able to undergo subsequent in-service weld repairs due to foreign object damage. While weld properties can be restored with full solution plus age post weld heat treatment, it is impractical to perform this operation due to possible airfoil distortion and surface contamination, especially for non-OEM welds. The current post weld heat treatment of 1100xc2x0 F. for 2-6 hours results in a very hard weld zone with low impact strength compared to parent metal and inadequate fatigue crack propagation capability. The post weld heat treatment could be raised to a 1300xc2x0 F. average temperature for up to two hours to restore acceptable weld zone impact and toughness properties; however, this treatment results in a 4-6% loss in tensile strength over the baseline condition. Such a loss is unacceptable for many highly stressed parts.
Accordingly, it is an object of the present invention to provide an improved method for fabricating and/or repairing integrally bladed rotors.
It is a further object of the present invention to provide a method which allows use of a high temperature post weld heat treatment while maintaining high tensile and fatigue strength.
It is a further object of the present invention to provide a novel encapsulated local airfoil heating device to perform said high temperature post weld heat treatment.
In accordance with a first aspect of the present invention, a method for creating an integrally bladed rotor broadly comprises providing a hub section, preferably formed from a titanium based alloy, and welding an airfoil, also preferably formed from a titanium based alloy, to the hub section. Prior to welding, the hub section and airfoil may be solution treated, oil quenched, partially aged and cooled prior to welding. The method further comprises applying a high temperature post weld average heat treatment to the weld joint between the hub section and the airfoil subsequent to welding.
A novel encapsulated local airfoil heating device is used to perform the post weld average heat treatment. The device broadly comprises a plurality of heating elements woven into a jacket made from a high temperature cloth material. The heating device is placed over the airfoil and the weld joint to perform the post weld heat treatment.
A method for repairing integrally bladed rotors in accordance with the present invention broadly comprises machining away a damaged portion of an integrally bladed rotor airfoil and welding an undamaged airfoil section to a remaining portion of the integrally bladed rotor airfoil. This is followed by placing the encapsulated local airfoil heating device over the undamaged airfoil and the weld and performing the post mold heat treatment to relieve residual stresses and restore microstructure and mechanical properties to the weld joint and adjacent metal.
Other details of the fabricating/repairing methods and the encapsulated local airfoil heating device, as well as other object and advantages attendant thereto, are set forth in the following description and the accompanying drawings wherein like reference numerals depict like elements.